Several chemical processes are known to produce an effluent (waste water) containing ammonium ions (NH.sub.4.sup.+), sulfate ions (SO.sub.4.sup.--) and organic substances. A typical example is the process for the manufacture of acrylonitrile (hereinafter referred to as AN) by the ammoxidation or of methyl methacrylate ester (hereinafter referred to as MMA). In these processes, the amounts of their respective effluents are invariably large for the unit yields of their products. In these effluents, ammonium sulfate and organic substances are contained at high concentrations. In the effluent which is discharged during the manufacture of AN, there are contained ammonium ions, cyanide ions (CN.sup.-), etc. in addition to the ammonium ions originating in ammonium sulfate. Release of this effluent in its unaltered form into a river or sea results in pollution. If this effluent is subjected in its unaltered form to combustive disposal and the resultant waste gas of combustion is released into the atmosphere, then the waste gas causes pollution of the atmospheric air.
Generally, the most reliable and simple method available for the disposal of any effluent containing organic substances in a high concentration is combustion. In the combustive disposal of the effluent supply of a large amount of auxiliary fuel is required where the effluent has no ample calorific value. In such a case, the effluent is deprived of as much of its water content as possible in advance such as by means of evaporation or desiccation and it is subsequently placed in a combustion furnace, so that it may be burned without use of the auxiliary fuel. Alternatively, it is necessary that the combustion of the effluent in its unaltered form be carried out by using such an auxiliary fuel in an irreducibly minimum amount. Where the effluent being delivered to the combustion furnace is in a form capable of being sprayed, the combustion is generally effected in a spray-type roasting furnace. A fluidized-bed type roaster is utilized where the effluent to be disposed is in the form of sludge, granule or powder, obtained by removing water from the effluent. Use of the fluidized bed type roaster also proves to be advantageous where the effluent to be disposed contains a large amount of water and other incombustible components and a low calorific value or it contains sparingly fusible inorganic substances. Substantially perfect combustion of organic substances is obtained by keeping the temperature of combustion generally above the level of 700.degree. C., preferably above 800.degree. C., and controlling the other conditions so that the gas discharged from the combustion of the effluent has a residual oxygen content of not less than 1.5%, preferably 3.0%, by volume, though all these conditions are variable to some extent with the type and size of the combustion furnace adopted.
A pollution problem may possibly result, however, when the effluent containing ammonium ions, sulfate ions and organic substances is treated in its unaltered form by the aforementioned method of combustion and the resultant waste gas is released into the atmospheric air. Combustion of the effluent in its unaltered form results in conversion of the ammonium ions present in the effluent to ammonia. If the combustion in this case is effected at a temperature of not higher than 1000.degree. C., then a part of this ammonia is oxidized as indicated by the reaction formula (1) given below. Nevertheless, a considerable amount of the ammonia remains intact in the waste gas of combustion. Part of the ammonia, upon exposure to oxygen, reacts as indicated by the reaction formula (2) below to produce nitrogen oxides (hereinafter referred to as NO.sub.x) at a temperature of higher than 1000.degree. C. EQU 2nh.sub.3 + 3/2 o.sub.2 .fwdarw. n.sub.2 + 3h.sub.2 o (1) EQU nh.sub.3 + o.sub.2 .fwdarw. no.sub.x + H.sub.2 O (2)
the waste gas of combustion, therefore, includes the NH.sub.3 and NO.sub.x thus formed. In effecting the combustive disposal of the effluent, therefore, ammonium ions must be removed before it is subjected to disposal. By proper selection of the operating conditions of the furnace alone, it is difficult to lower successfully the undecomposed NH.sub.3 and NO.sub.x contents of the waste gas of combustion to levels tolerable for release into the air, say to less than 100 ppm respectively. The SO.sub.4.sup.-- which is present in the effluent is thermally decomposed into sulfur dioxide (SO.sub.2) when it is heated to a temperature of 800.degree. C. or over. At such a high temperature, an equilibrium indicated by the formula (3) below is established between the sulfur oxides SO.sub.2 and SO.sub.3. EQU so.sub.3 .revreaction. so.sub.2 + 1/2 o.sub.2 ( 3)
at temperatures above the level of 800.degree. C., the equilibrium expressed by the formula (3) above shifts substantially in the direction of the righthand member of the formula. The waste gas of combustion which consequently contains the produced SO.sub.2 naturally causes air pollution unless the SO.sub.2 content thereof is lowered to a sufficiently tolerable level such as, for example, to less than 100 ppm by volume before the gas is released into the air. In order that such occurrence of SO.sub.2 within the roaster may be prevented in the combustive disposal of the effluent, therefore, it is necessary that the SO.sub.4.sup.-- be removed prior to the combustion of the effluent or it be properly treated to have the SO.sub.4.sup.-- converted to an inorganic salt incapable of producing SO.sub.2 through thermal decomposition upon exposure to an elevated temperature.